Ch. 15 & 16 - Acids & Bases
II. pH
(p. 481 - 491)
C. Johannesson
A. Auto-Ionization of Water
 Water molecules react with other water molecules
to IONIZE (make ions).
-
+
H3
H2O + H 2O
base
acid
LPChem:Wz
+
O
+
OH
A. Ionization of Water
H 2O + H 2 O
H3
+
O
+
OH
 “Pure” water contains hydronium and
hydroxide ions.
 Note the equilibrium arrow!
 Auto-ionization only happens to one out of every
quarter-billion molecules, so water doesn’t contain
enough ions to be an electrolyte.
LPChem:Wz
A. Ionization of Water
H 2O + H 2 O
H3
+
O
+
OH
 The product of the concentrations of H3O+ and OHin aqueous solution will always equal the equilibrium
constant, Kw.
Kw = [H3O+][OH-] = 1.0  10-14
C. Johannesson
A. Ionization of Water
 Find
the hydroxide ion concentration of
3.0  10-2 M HCl.
HCl is a
strong acid,
[HCl] =
[H3O+]
[H3O+][OH-] = 1.0  10-14
[3.0  10-2][OH-] = 1.0  10-14
[OH-] = 3.3  10-13 M
Acidic or basic? Acidic
C. Johannesson
[H3O+] > 1x10-7
Or
[H3O+] > [OH-]
 acidic
A. Ionization of Water
 Find
the hydronium ion concentration of
2.5  10-5 M NaOH.
NaOH is a
strong
base,
[NaOH]
= [OH-]
[H3O+][OH-] = 1.0  10-14
[H3O+][2.5  10-5]= 1.0  10-14
[H3O+]
= 4.0 
10-10
M
Acidic or basic? Basic
[H3O+] < 1x10-7
Or , [H3O+] < [OH-]
C. Johannesson
Write answers
in Scientific
Notation!
B. pH Scale
14
0
7
INCREASING
ACIDITY
pH =
NEUTRAL
+
-log[H3O ]
pouvoir hydrogène (Fr.)
“hydrogen power”
C. Johannesson
INCREASING
BASICITY
B. pH Scale
pH of Common Substances
C. Johannesson
B. pH Scale
pH =
+
-log[H3O ]
pOH =
-log[OH ]
pH + pOH = 14
C. Johannesson
B. pH Calculations
[H+]
(H3O+)
-log[H3O+]
pH
10^(–pH)
14 = pH + pOH
Kw = [H3O+][OH-]
-log[OH-]
[OH-]
pOH
10^(–pOH)
LPChem:Wz
B. pH Scale
 What
is the pH of 0.050 M HNO3?
pH = -log[H3O+]
strong acid 
[HNO3] = [H3O+]
__  2 sig figs
pH = -log[0.050]
pH = 1.30
__  2 sig figs
after the decimal
Acidic or basic? Acidic
C. Johannesson
pH < 7
B. pH Scale
is the pH of a 1x10-9 M HNO3
solution?
 What

HNO3 is a strong acid

Therefore [HNO3] = [H3O+]

pH = -log[H3O+]

pH = -log[1x10-9] = 9.0
LPChem:Wz
Basic
B. pH Scale
is the [H3O+] of a solution with pH
= 7.45?
 What

pH = -log[H3O+]

Therefore 10^ -pH = [H3O+]

10^ -7.45 = [H3O+]

= 3.6 x 10-8 M
LPChem:Wz
B. pH Scale
 What
is the molarity of HBr in a solution
that has a pOH of 9.6?




Given: pOH
Find:
Molarity of HBr
HBr is a: Strong Acid
[HBr] = [H3O+]
How do we solve from pOH to [H3O+] ?
C. Johannesson
B. pH Calculations
[H+]
(H3O+)
-log[H3O+]
pH
10^(–pH)
14 = pH + pOH
Kw = [H3O+][OH-]
-log[OH-]
[OH-]
pOH
10^(–pOH)
LPChem:Wz
B. pH Scale
 What
is the molarity of HBr in a solution
that has a pOH of 9.6?
pH + pOH = 14
pH = -log[H3O+] or
pH + 9.6 = 14
10^ -pH = [H3O+]
pH = 4.4
10^ -4.4 = [H3O+]
Acidic
= 4.0  10-5 M HBr
C. Johannesson
B. pH Scale
is the OH- concentration of a
solution whose pH is 1.94?
 What
 Given: pH
 Find:
[OH-]
How do we solve from pH to [OH-] ?
C. Johannesson
B. pH Calculations
[H+]
(H3O+)
-log[H3O+]
pH
10^(–pH)
14 = pH + pOH
Kw = [H3O+][OH-]
-log[OH-]
[OH-]
pOH
10^(–pOH)
LPChem:Wz
B. pH Scale
is the OH- concentration of a
solution whose pH is 1.94?
 What
pH + pOH = 14
pOH = -log[OH-] or
1.94 + pOH = 14
10^ -pOH = [OH-]
pOH = 12.06
10^ -12.06 = [OH-]
Acidic
= 8.7  10-13 M OHLPChem:Wz
B. pH Scale
 What
is the pH of a 0.055 M KOH
solution?
 Given: [OH-]
 Find:
pH
How do we solve from [OH-] to pH?
C. Johannesson
B. pH Calculations
[H+]
(H3O+)
-log[H3O+]
pH
10^(–pH)
14 = pH + pOH
Kw = [H3O+][OH-]
-log[OH-]
[OH-]
pOH
10^(–pOH)
LPChem:Wz
B. pH Scale
 What
is the pH of a 0.055 M KOH
solution?

KOH is a strong base

Therefore [KOH] = [OH-]

pOH = -log[OH-]

pOH = -log[0.055]

pOH = 1.26
pH + pOH = 14
pH + 1.26 = 14
pH = 12.74
Basic
LPChem:Wz